actually, the G5s were very, very quiet compared to the mirror door G4s before them. Those things literally sound like a jet taking off. The worst part was that they would get louder and softer without any correlation to the amount of work they were doing. My wife had one in her office, and it was the worst thing to try and have a phone conversation and have to wait for your computer's fans to slow down for a minute so you could hear...
I've never experienced a louder computer that wasn't broken.
The Mac Pros are quieter than the G5s with one exception: The optical drive is LOUD. If there is a disc in the drive, it's as loud as a mirror-door G4. If I want to watch a DVD, I have to rip it, because I can't hear the sound over the spinning of the disc.
The rest of the time, it's silent, though.
I guess that's because I only have the 7300. I hear the 1900s at the apple store, and I'm actually glad I don't have one. Good to hear that the 8800 is quiet. Bad to hear that it isn't compatible with 2-week-old mac pros...
but that's a different thread.